osd/: clarify rebuilt_missing_with_deletes and set in init

osd/: clarify rebuilt_missing_with_deletes and set in init

Updates PeeringState::init to set may_include_deletes_in_missing when
possible to avoid needlessly rebuilding the missing set on the next map.
The previous behavior was also subtly wrong as until the the next map
the osd would be possibly adding deletes to the missing set without the
flag having been set in the missing set on disk

This patch also renames the flag to reflect the fact that it simply
denotes wheter the may_include_deletes_in_missing omap entry needs to be
written out.  This may happen during rebuild_missing_set_with_deletes,
but it'll also happen more commonly during init now.

Signed-off-by: Samuel Just <sjust@redhat.com>